Small strongyles
Pathogenesis - larval stages
The larvae of most cyathostome species penetrate the gut wall as far as the mucosa but no
further, but a few species will go as far as the muscularis mucosa. However deep the
penetration, all larvae become encapsulated in small nodules in which they continue their
development. These nodules are readily visible at necropsy and may be quite numerous
- Poynter has recorded as many as 60 nodules per square centimeter of mucosa
in work published in 1970 and others have recorded similar numbers. However, with the use
of more effective anthelmintics in parasite control programs far fewer nodules have
been seen at necropsies in more recent times.
The size and location of nodules appears to depend on the species
involved. For example Cylicostephanus species appear to develop in the mucosa
producing small red to blackish nodules 2-3mm in diameter. Cylicocyclus and
Gyalocephalus species penetrate to the submucosa and provoke larger nodules
anywhere from 3-6mm in diameter.
Mucosal dwelling larvae
become surrounded by fibroblasts. Histological sections will show goblet cell
hypertrophy and hyperplasia around the capsule with an accompanying infiltration of
lymphocytes and eosinophils. The latter becomes particularly intense as the larvae emerge
from their capsules to reenter the gut lumen.
Larvae in the submucosa
provoke a similar but more intense reaction and the capsule is usually thicker. In horses
with light infections, pathological changes tend to be restricted to the
encapsulated larvae and the immediately surrounding areas. In heavy infections the
pathological changes are correspondingly more serious - lesions become coalesced and
result in a generalized disruption of the mucosa, a more intense cellular infiltration,
edema and copious production of mucus. Surface ulcerations may be common and result from
larvae escaping from their nodules.
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Encapsulated small strongyle larvae in the cecal wall
as seen at necropsy.
Image courtesy of Professor Max Murray |
Encapsulated small strongyle larvae shown via
trans-mucosal illumination.
Image courtesy of Dr. J.H. Drudge and Hoechst Roussel. |
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A close-up view of encapsulated small strongyle larvae
in the cecal wall.
Image courtesy of Merial Inc |
Histological section of the large intestine showing an encapsulated
small strongyle larva in the mucosa
Image courtesy of Dr A. O. Mathison |
Pathological changes tend to be more severe in equines less than two years old. In
these animals, heavy infections tend to produce gross changes in the cecum and colon
that are characterized by hemorrhagic, catarrhal and fibrinous enteritis. The mucosa tends
to be thickened and edematous with varying numbers of ulcers produced by larvae escaping
from their nodules. The color of the mucosa varies from a normal greyish pink with
varying numbers of small hemorrhages to a dark red in severe infections. Mucus production
tends to be significant and some areas of the mucosa may have fibrinous deposits.
Pathogenesis - adult stages
Pathological changes due to adult small strongyles tend to be less severe than
those due to either their larval stages or adult large strongyles. Their shallow buccal
capsules mean that they feed mainly by grazing on surface areas of the mucosa in
contrast to the larger bites taken by the large strongyles. However, small
strongyles tend to be present in much greater numbers than large strongyles so their
cumulative effects can be more serious especially where parasite control programs are
poorly implemented or ineffective. However, larval migrations of Strongylus vulgaris
are potentially more pathogenic because of their migrations through the mesenteric
arterial system.
